Hydrocarbons such as oil, natural gas, etc. are obtained from a subterranean geologic formation (e.g. a “reservoir”) by drilling a well that penetrates the hydrocarbon-bearing formation. This provides a partial flowpath for the hydrocarbon, typically oil, to reach the surface. In order for oil to be “produced”, that is, travel from the formation to the wellbore (and ultimately to the surface), there must be a sufficiently unimpeded flowpath through the formation rock (e.g. sandstone, carbonates), which generally occurs when rock pores of sufficient size and number are present.
A common reason for a decline in oil production is “damage” to the formation, which plugs the rock pores and impedes the flow of oil. Often such damage can be attributed to a number of factors including, the methods and chemicals used in establishing the well, remedial operations performed on the well, or the formation being naturally “tight” (e.g. a low permeability formation), with pores sufficiently small that the oil migrates toward the wellbore only very slowly.
Generally, techniques used to increase the permeability of the formation are referred to as “stimulation”. Stimulation of the formation can be performed by: (1) injecting chemicals into the wellbore to react with and/or dissolve damage; (2) injecting chemicals through the wellbore and into the formation to react with and/or dissolve small portions of the formation to create alternative flowpaths for the hydrocarbon; or (3) injecting chemicals through the wellbore and into the formation at pressures sufficient to fracture the formation, thereby creating a channel through which hydrocarbon can more readily flow from the formation and into the wellbore.
Hydraulic fracturing involves breaking or fracturing a portion of the surrounding strata of the formation, by injecting a specialised fluid into the wellbore directed at the face of the formation at pressures sufficient to initiate and extend a fracture in the formation. Typically, the process creates a fracture zone, that is, a zone in the formation having multiple fractures, through which hydrocarbon can more easily flow to the wellbore.
Typical fracturing treatments e.g. fluids, generally comprise at least three components; a carrier fluid (usually water or brine), a polymer, and a proppant. Many further comprise a crosslinker. Other compositions used as fracturing fluids include water with additives, and gelled oils. The purpose of these fracturing fluids is to firstly create and extend a fracture, and then once it is opened sufficiently, deliver proppant into the fracture via the carrier fluid, which keeps the fracture from closing once the pumping operation is completed.
Viscoelastic compositions have also been found to be usefully employed as fracturing fluids. Conveniently, use has been made of surfactants which when in an aqueous solution are capable of forming a viscoelastic composition for this purpose. Such surfactants are referred to herein for brevity and simplicity as “viscoelastic surfactants”. The utility of fracturing fluids comprising viscoelastic surfactants has been attributed to the Theological properties of the fluid compositions, the stability of such fluids and their low residue content.
Conventional surfactants, specifically those which tend to form spherical micelles, are generally not capable of forming a viscoelastic composition, particularly an aqueous viscoelastic composition, and are thus not suitable for use in a hydraulic fracturing application. However, certain surfactants, specifically those which tend to form long rod-like or worm-like micelle structures, e.g. viscoelastic surfactants, are capable of forming an aqueous viscoelastic composition which is readily applicable in hydraulic fracturing. At a relatively low total concentration of a viscoelastic surfactant, typically in the range 1 to 10 wt %, these long rod-like or worm-like micelle structures overlap, forming an entangled network which is viscoelastic. Typically, these large micelle structures are readily destroyed by their interaction with formation fluids such as hydrocarbon fluids. When the micellar structures are broken by their interaction with the hydrocarbon fluid, a solution with low viscosity is formed. Thus, as the viscoelastic surfactant based fracturing fluid interacts with produced hydrocarbon fluids, a dramatic change in micellar structure (from rod-like or worm-like to spherical micelles) for instance causes a dramatic change in the rheological properties of the fracturing fluid (from a viscoelastic composition to an inviscid solution). It is this “responsive” fluid which facilitates easy removal and clean up of the fluid from the propped fracture so as to maximise hydrocarbon production.
The application of viscoelastic surfactants in both non-foamed and foamed fracturing fluids has been described in several patent specifications.
U.S. Pat. No. 5,258,137 relates to foam fluid compositions, which are described as stable over a range of temperatures, easily formulated and possessing good shear stability, and which comprise an aqueous liquid, a thickening amount of a viscoelastic surfactant e.g. including those represented by the following formula.
and a functionally effective amount of a surfactant which is capable of forming a foam.
U.S. Pat. No. 5,551,516 describes a hydraulic fracturing fluid comprising an aqueous base fluid e.g. water, a thickener selected from a specified group, an inorganic water soluble salt, at least one viscoelastic surfactant for suspending a proppant during placement, and a stabilising organic salt or C4 to C12 alcohol. The fracturing fluid is stated to find application in the fracturing treatment of high permeability subterranean formations.
U.S. Pat. No. 5,964,295 describes methods for, (i) reducing fracturing fluid loss into a relatively low permeability formation during fracturing, (ii) enhancing the cleanup of a fracturing fluid from a well and reducing the production of water from a subterranean formation, and (iii) reducing the equipment required to mix and pump fracturing fluids, by employing a fracturing fluid containing a viscoelastic surfactant. In a further described method, an aqueous viscoelastic surfactant based hydraulic fracturing fluid comprising an aqueous based thickener, a water soluble salt, and at least one amine or salt of an amine thickener, is used to fracture a formation.
U.S. Pat. No. 5,979,557 relates to a method for acidizing a formation, and to a method for limiting the inflow of formation water during and after a well turn around, to maximise recovery of the hydrocarbons and fracturing fluid, the methods comprising a step of, selectively blocking the pore structure of the formation face in the water-bearing zone, but not in the hydrocarbon zone. In a preferred embodiment, the pore structure is blocked by a plug of viscous fluid, which comprises amongst other components, a viscoelastic surfactant which is capable of forming worm-like micelles in an aqueous environment.
A potential disadvantage associated with the use of the viscoelastic surfactants of the prior art, is the tendency of the individual viscoelastic surfactant molecules to form emulsions with the formation fluid (i.e. the hydrocarbon to be extracted) following fracturing. Emulsion droplets formed within the fracture or within the invaded matrix zones may produce a barrier to formation fluid flow which may limit fluid clean up and hydrocarbon production.